In recent years, there has been a remarkable spread of mobile phones and PDAs (Personal Digital Assistant). It has become a common practice to incorporate a compact digital still camera unit and digital video unit in such devices. These devices are characterized by severe restrictions on the size and cost. Therefore, there has been generally used a small-sized image pickup device equipped with a smaller number of pixels than that in a normal digital still camera, and a unifocal optical system including about one to three plastic lenses. In the image pickup device mounted on a mobile information terminal, technology is advancing rapidly toward a further increase in pixel level and functional level. In this context, there has been an intense demand for a compact variable-power optical system compatible with an image pickup element of high pixel level and capable of capturing the image of a subject located far away from a user, wherein this variable-power optical system can be mounted on a mobile phone and others.
For example, the Patent Literature 1 discloses a three-component variable-power optical system with negative-positive-negative components wherein the first lens group is fixed to achieve minimizing the optical system. In the disclosed optical system, however, a power arrangement in the first lens group and lens materials in the second lens group are not optimized, and correction of astigmatism and chromatic aberration becomes insufficient.
The Patent Literature 2 proposes a four-component variable-power optical system with negative-positive-negative-positive components, a so-called light bending optical system wherein the optical axis is bent at right angles and the optical unit is designed to have a slim configuration. However, this system contains a prism for bending the optical axis, and this results in increased costs. Although an attempt for thin configuration has been made, the overall length of the optical system is excessive, and efforts toward compact configuration are insufficient in terms of unit volume.
The Patent Literature 3 proposes a four-component variable-power optical system with negative-positive-negative-positive components wherein all four lenses are movable to ensure both of an increased angle of view and back-focusing. However, this arrangement entails the disadvantages of an increase in the number of movable lens groups, and insufficient correction of spherical aberration and astigmatism. More compact configuration of the variable-power optical system is difficult.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2005-55725
Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2004-205796
Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2001-116992